Is it even fair to call that mediocrity?

Maybe it's "cerebral suppression" where you intentionally switch off the part of your brain that wants to make it more clever and smarter.

Chuck Lorre (creator of Two and a Half Men and Big Bang Theory) comes to mind.

I present to you, ladies & gents, James Cameron:

https://en.wikipedia.org/wiki/James_Cameron

Avatar and Titanic are stunningly mediocre films. But look at the box office numbers.

Cameron is strange because he is a legitimate genius at some parts of movie-making. For all their mediocrity, Titanic and Avatar have exceptional action sequences - and Cameron is a bonafide genius at that.

One of the remarkable things about online commentary: that an achievement like Titanic can be dismissed as mediocrity! Is it sophisticated critique of potboiler entertainment or mere ressentiment in the comments sections? Either way, Cameron is exceptional, if only from the standard of commercial success.

To be fair he also created The Terminator, Aliens and Terminator 2: Judgement Day—all of these films were blockbusters and arguably excellent films.

Avatar and Titanic presumably aspired to be excellent films as well. The visual effects that went into each were nuts, and Titanic's production values were quite good.

Why legendary directors sometimes end up making progressively worse films later in life is a rather complex question.

I'm not arguing, but what are some examples?

I'll start: Spielberg.

I would argue with this, though: "Avatar and Titanic presumably aspired to be excellent films as well."

No, I don't think they did. I think by now Cameron had internalized the screenwriter's gospel of character arcs and what audiences (presumably) want to see, and regurgitated it. "Excellent" to him meant "what people will pay for."

Somewhere in there I think is ego gone awry; why write for what you think audiences want to see? That's such a cynical corruption of art.

I suppose it's fear of your movie gross failing to exceed your production budget, and you retire as a legendary director anyways. Ah, that doesn't seem too bad. Well worth the risk. But, no.